It's the boiling that requires no plugs, or coating of the bores. Not being argumentative, and you can't hurt anything in doing so, but it's a waste of time and effort. Boiling water can't hurt anything, this according to Angier and personal experience.
When it comes to the etching tank, something must be done to protect the bores. I had considered using plugs, but got to thinking "what if one leaks a bit?" At that point, I decided I would rather go through the trouble of coating. Done in this fashion, I'm certain the etchant never meets the bores.
My shop is in the basement, approximately 70 deg. year 'round. 15% does quite well, or has so far, but I do keep in mind that with some metals, immersion time or bath strength may need to be altered.
I use a rusting cabinet. Actually, you might call mine an environmental chamber. My day job involves HVAC and refrigeration work. I can control heat and humidity fairly precisely in the contraption I've built. On a Saturday or Sunday,[when I'm there all day], I can get in 4 or so good rustings.
Once again, the premise is the same, but slightly different approaches to skinning the same cat.